1. Field of the Invention
This invention relates to a measuring device for determining the displacement of a movable object in cooperation with a graduation element connected to the object and having at least a track which is formed from a plurality of successive grating strips extending transversely to the direction of movement of the object, said measuring device comprising at least a radiation source for generating radiation and an associated optical system for converting the radiation into a radiation beam which is parallel on average and for guiding this radiation beam towards the graduation element, and a radiation-sensitive detection system for converting radiation from the graduation element into an electric signal.
2. Description of the Prior Art
Patent measuring device of the type described in the opening paragraph is known from U.S. Pat. No. 3,973,119. In the measuring device described in this Patent a grating mechanically connected to the object is projected on a multiple photocell detector. This detector comprises a row of strip-shaped, substantially identical photosensitive cells which are consecutively connected by means of an electric switch to an electric circuit which processes the signal generated in the photosensitive cells. The electric switch can ensure that the detector may serve as a reference grating. The grating is preferably imaged 1:1 on the photosensitive cell array. To introduce a minimum number of read errors in such an image due to oblique projection of the grating strips on the detector, the radiation from the radiation source is to be converted into a scanning beam which is parallel in the cross-section perpendicular to the grating strips. Parallelism is not required in the cross-section parallel to the grating strips.
In the measuring device described in said United States Patent, the radiation from the radiation source is guided towards a collimator lens via a partially transparent mirror. The parallel scanning beam formed by the collimator lens is subsequently incident on the graduation element. After reflection on the graduation element, the beam travels to the partially transparent mirror via the collimator lens so that the beam is reflected towards the detector.
The known measuring device has the drawback that the optical system for generating a parallel scanning beam is a combination of standard lenses and mirrors and consequently occupies a relatively large space. A further drawback is the low light output. The partially transparent mirror only passes approximately half the radiation incident thereon, so that only approximately a quarter of the radiation emitted by the radiation source reaches the detector.